1. Field of the Invention
This invention relates to a liquid composition for treatment of the eyes. The liquid eye drop composition may-be used to treat irritations of the eye, dryness of the eye, and/or the onset or progression of cataracts.
2. Discussion of Related Art
The ocular lens of the eye is a relatively pliable and normally transparent tissue held in suspension between the aqueous and vitreous humors. The tissue has a fibrous type structure. The lens functions as a converging lens, in conjunction with the cornea, pupil, aqueous humor and vitreous humor. The pliability and transparency of the lens are necessary for the proper functioning of the lens in image formation and visual perception.
The lens of the eye is constantly exposed to light and ambient oxygen, and is thus very susceptible to oxidation, particularly light-induced lipid peroxidation. The lens has a complex biochemical system, well understood in the art, relying upon proteins, particularly proteins containing sulfhydryl groups, to maintain the system and in turn the transparency of the lens.
Light entering the eye permits the generation of free radicals within the lens, in particular the superoxide radical O.sub.2.sup.-, which in turn can degenerate into other free radicals such as hydrogen peroxide and hydroxide radicals. These free radicals act to oxidize the proteins of the lens. Oxidation of the proteins is known to be a major factor leading to the onset of cataracts, which is a loss of transparency of the lens. See, for example, Varma, "Scientific Basis for Medical Therapy of Cataracts by Antioxidants", Am. S. Clin. Nutr., vol. 53, pps. 335S-345S (1991).
Accordingly, the lens of the eye has an antioxidant defense system to respond to an oxidative stress and maintain the integrity of the lens. Various studies have shown that the antioxidant defense system includes the enzymes glutathione peroxidase, catalase and superoxide dismutase, and the antioxidants vitamin A (ascorbic acid), vitamin E (.alpha.-tocopherol) and .beta.-carotene. See, for example, Kamei, "Glutathione Levels of the Human Crystalline Lens in Aging", Biol. Pharm. Bull., vol. 16, no. 9, pps. 870-875 (1993); Fletcher et al., "Glutathione and Aging: Ideas and Evidence", The Lancet, vol. 344, pps. 1379-1380 (1994); and Jacques et al., "Antioxidant Status in Persons With and Without Senile Cataracts", Arch. Opthalmol., vol. 106, pps. 337-340 (1988).
The role of each of the above materials in protecting the lens against degradation by oxidation has also been widely studied. Most studies have focused upon dietary supplementation of the materials to preserve the antioxidant defense system of the lens during aging, thus preventing or slowing the onset of cataracts. See, for example, Robertson et al., "Vitamin E Intake and Risk of Cataracts in Humans", Annals New York Academy of Sciences, pps. 372-382; "Protective Role of Vitamin E in Cataract Development", Vitamin E Research Information Service (1990); Devamanoharan et al., "Prevention of Selenite Cataract by Vitamin C", Exp. Eye Res., vol. 52, pps. 563-568 (1991); and Jacques et al., "Epidemiologic Evidence of a Role for the Antioxidant Vitamins and Cartenoids in Cataract Prevention", Am. J. Clin. Nutr., vol. 53, pps. 352S-355S (1991). See also U.S. Pat. No. 5,075,116 to LaHaye et al.
Studies have also confirmed that the materials vitamin A, vitamin E and glutathione have a close interaction in regenerating one another following oxidation of one or more of these molecules. See, for example, Stoyanovsky et al., "Endogenous Ascorbate Regenerates Vitamin E in the Retina Directly and in Combination With Exogenous Dihydrolipoic Acid", Current Eye Research (1994); and Winkler et al., "The Redox Couple Between Glutathione and Ascorbic Acid: A Chemical and Physiological Perspective", Free Radical Biology & Medicine, vol. 17, no. 4, pps. 333-349 (1994).
A role has also been reported for zinc and copper in maintenance of retinal metabolism. See Hirayama, "Histochemical Localization of Zinc and Copper in Rat Ocular Tissues", Acta Histochem., vol. 89, pps. 107-111 (1990). Zinc has been cited as a cofactor for several antioxidant systems present in the retinal pigment epithelium. See Newsome et al., "Zinc Uptake by Primate Retinal Pigment Epithelium and Choroid", Current Eye Research, vol. 11, no. 3, pps. 213-217 (1992).
Although extensive reporting has been done on the significance of dietary supplementation of various lens antioxidant system materials in maintaining the integrity of the defense system in times of oxidative stress and throughout aging, there is little discussion related to eye drop formulations that combat oxidation in the lens. One exception is Cataract Breakthrough, The Booklet, by Dr. Alex Duarte, discussing research in which the three amino acids cysteine, glycine and glutamic acid, the amino acids that make up and synthesize glutathione, were introduced to the lens through eye drops. Dr. Duarte recommends a cataract prevention treatment involving these eye drops in conjunction with dietary supplementation of all antioxidant defense materials.